1. Field of the Invention
The present invention relates to a heat storage mat of the type using a substance which is capable of evolving heat at a constant temperature when the substance undergoes a phase change or transformation from the liquid state into the solid state. The heat storage mat is particularly useful when embodied in a napping or brief sleeping bed for long-distance trucks. When the truck is running, the heat storage mat is able to store heat by using electric power which is generated while a truck is running, and when the driver is taking a nap within the truck at a stop, the heat storage mat evolves heat so as to keep the inside of the napping bed at a comfortable temperature.
2. Description of the Prior Art
One known heat storage mat is disclosed in Japanese Utility Model Laid-open Publication 2-42266, which utilizes a latent heat storage substance or agent capable of evolving heat at a transformation temperature. Electric power which is generated while a truck is running is used in order to store or absorb heat within the heat storage agent. When the driver is taking a nap on the heat storage mat, the heat storage agent evolves heat to warm up the body of the driver in such a manner that the feet are kept at a higher temperature than the upper half of the body.
The known heat storage mat includes a heat insulating cushion wrapped in a bag-like cover and containing therein a series of heating elements. Each of the heating elements is composed of a bag filled with a heat storage agent and an electric heater unit that are disposed in a superposed relation within the heat-insulating cushion. When the mat is not used, the electric heater unit is energized to heat the heat storage agent which in turn absorbs heat as latent heat. When the electric heater unit is de-energized, the heat storage agent starts evolving heat to warm up the mat so that the mat is kept at a predetermined temperature for a predetermined period of time. The heat storage mat thus constructed is able to operate without the need for a battery used as a heat source, so that during the winter months or at a cold district, the truck driver can take a nap on the mat while the engine is stopped. Thus, over-discharging of the battery can be avoided.
In the heat storage mat described above, the heat storage agents received in the respective bags have different transformation temperatures so that the heating elements generate heat at different temperatures. The heating elements are arranged to create a temperature gradient so that the foot side of the truck driver is heated at a higher temperature than the head side. With this temperature gradient, the mat is able to provide a comfortable sleeping condition for the truck driver.
The known heat storage mat, however, is not fully satisfactory for the reasons discussed below in greater detail with reference to the accompanying drawings.
As shown in FIG. 3, the known heat storage mat B is composed of four parts B1-B4 arranged in a row in order to support thereon corresponding parts of the driver's body A. More specifically, the first part B1 is adapted to support a head A1 and shoulders A2 of the body A; the second part B2 is adapted to support a chest A3 and a waist A4 of the body A; the third part B3 is adapted to support a hip A5 and thighs A6 of the body A; and the fourth part B4 is adapted to support legs A7 of the body A. When the driver takes a nap on the heat storage mat B, most parts of the body A excluding the head A1 are covered with a coverlet such as a blanket C.
Various conditions which are required for the respective parts B1-B4 to keep the corresponding parts of the body A in a comfortable condition are as follows.
The head A1 which is placed on a pillow D requires a greater heat radiation rather than a heat retaining property. The shoulders A2 evolve a relatively small amount of heat but they are not so thermally weak to require an external heat source. In addition, a front side of the shoulders A2 is thermally insulated by the blanket C. The back of the shoulders A2 requires a high thermal insulation and an excellent cushioning effect.
The chest A3 and the waist A4 give off a great amount of heat so that when wrapped with the blanket C, these parts can be kept warm without using a heat source. However, to insure comfortable napping, the chest A3 and the waist A4 require an excellent cushioning effect.
The hip A5 and the thighs A6 evolve less amount of heat than the chest A3 and the waist A4 and hence these parts are unlikely to become warm. Thus, these parts require a heat source of a large thermal capacity. The back of the hip A5 and the thighs A6 is insensitive as compared with the back A3 and lower back A4. Accordingly, an excellent cushioning effect is not needed for the back of the hip A and the thighs A6.
The legs A7 evolve the least amount of heat and release a greater amount of heat. Accordingly, the legs A7 are likely to become cold soon. In order to warm up the legs A7, a greater heat source must be provided. In addition, since the legs A7 are likely to move frequently and widely, a heat retaining effect of the blanket C is substantially deteriorated. The cushioning property of the upper side of the fourth part B4 does not have a direct effect on a comfortable sleep.
The known heat storage mat failed to satisfy all of the requirements discussed above.